The present invention relates to thermoplastic high-Tg polycarbonates and moulding materials which are distinguished by improved thermal properties and improved mechanical properties, in particular by reduced thermal expansion. The present invention furthermore relates to a process for the preparation of these polycarbonates. In particular, this invention relates to polycarbonates which the structural unit which derives from phthalimide of the formula (I) and polycarbonate compositions and moulding materials therefrom as well as a process for the preparation of these polycarbonates, and the use thereof, in particular as reflectors and display substrates.
(Co)polycarbonates belong to the group consisting of the industrial thermoplastics. They have a wide range of uses in the electrical and electronic sector, as housing material of lights and in applications where both particular thermal and mechanical properties and outstanding optical properties are required, for example applications in the automotive sector, plastic covers, diffuser screens or waveguide elements and lamp covers, lamp bezels. Reflectors or subreflectors produced from a thermoplastic by injection moulding with outstanding surface quality would have been a very interesting addition to these applications for (co)polycarbonates.
In these applications, the good thermal properties such as Vicat temperature (heat distortion resistance) and glass transition temperature, are virtually always essential. Good adhesion to metal, for example to aluminium, is also indispensable for some applications. At the same time, outstanding optical properties are of the greatest importance. To date, mechanical properties with regard to thermal expansion of the amorphous polycarbonate with constant thermal properties, such as the coefficient of linear thermal expansion, remain unconsidered.
Thermoplastics from which light-reflecting components are produced by injection moulding and subsequent metallization (vacuum coating, generally with aluminium) are known. Such components are, for example, headlamp reflectors for automobiles. In addition to the paraboloid headlamps previously used without exception, two basic types optimized with regard to light utilization and space requirement were developed, the projection headlamp (ellipsoid, polyellipsoid) and the free-surface headlamp. Since, owing to the optimized light utilization and distribution of this reflector type, the lenses, in particular of free-surface headlamps can generally be designed without a profile, clear polycarbonate lenses are used today. This increases the requirements regarding the surface quality of the elements clearly visible from the outside (e.g. reflector, subreflector, frame), the dimensional stability at elevated temperature, little gas emission to avoid bubble formation, the mechanical strength, easy processing and low manufacturing tolerances furthermore being important.
To date, headlamp reflectors have been produced either from sheet metal or metallized injection moulded parts comprising thermosetting plastic (bulk moulding compounds, BMC). Good dimensional stability and thermal stability are required here.
Headlamp reflectors can also be divided into the actual reflector essentially having a paraboloid shape and a subreflector differing to a greater or lesser extent from the paraboloid shape. The reflector is the actual component which reflects the light in a targeted manner for the desired illumination and which is usually arranged in the immediate vicinity of the light-producing incandescent lamp. The lamp or incandescent bulb or light source corresponding to these produces not only light but also heat so that the reflector may be exposed to an operating temperature of about 180-220° C., depending on design. For this reason, it is necessary to provide materials having a coefficient of linear thermal expansion as low as possible. This material should be capable of being processed as far as possible by injection moulding technology and should be economical.
In addition, the reflectors should be dimensionally stable in a temperature range from −50° C. to 220° C., i.e. the expansion and shrinkage behaviour must as far as possible be isotropic so that—at least in the case of the reflector—the luminous efficiency or light focussing is not adversely affected. Preferably, the metal layers have substantially the same expansion and shrinkage behaviour as the reflectors, so that the tensile or shear stress of the reflection layers is as small as possible. As a result, the danger of cracking or compression in the reflection layers is additionally reduced.
In general, thermosetting plastics, more rarely also thermoplastics, have been used to date for producing reflectors. Of the latter, the amorphous thermoplastics mainly used, e.g. polyetherimide (PEI), polyamidimide (PAI) or polysulphones, e.g. polyether sulphone (PES) or polysulphone (PSU) or polyphenylene ether sulphone (PPSU), have a high to very high glass transition temperature (Tg) (cf. for example PAI TORLON® from Solvay Advanced Polymers). These amorphous high-Tg thermoplastics can be used without fillers for producing reflector blanks having outstanding surface smoothness. Reflector blanks can be directly metallized. A disadvantage for mass production is, however, the very high cost of said amorphous high-Tg thermoplastics in some cases. Moreover, the processing of some of these high-Tg thermoplastics is difficult.
For headlamp reflectors, mainly bulk moulding compounds (BMC) have been used for some time. These are the semifinished fibre-matrix product. It generally consists of short glass fibres and a polyester or vinyl ester resin; other reinforcing fibres or resin systems are possible. BMC is processed in the hot pressing process, which permits short cycle times. For this purpose, the BMC material is inserted centrally into a heated, divided mould. On closing, the BMC is distributed in the mould. Owing to the short fibre lengths, thin ribs and wall thicknesses can also be filled during pressing. However, there is the danger that the BMC will separate at constrictions. This occurs when a constriction is blocked by fibres so that only the resin can flow onwards. The individual reinforcing fibres are as a rule oriented in the direction of flow, so that locally highly oriented fibres may occur. In special processes, BMC, with appropriately small fibre lengths, can also be processed in the injection moulding process.
A typical application for thermosetting plastics (BMC) comprises car headlamps, more precisely the reflectors of the headlamp. The good dimension stability and thermal stability play a role here. The process resembles to a very great extent elastomer injection moulding. The cycle times in the processing of thermosetting plastics is as a rule longer than in the case of thermoplastics at wall thicknesses up to about 4 mm. As a result, thermosetting plastics are generally inferior to the thermoplastics in a cost-efficiency comparison if the good electrical and mechanical properties are not required.
The fillers predominantly perform the function of producing the BMC more economically in that fibre and resin volume is replaced by cheaper fillers. Depending on the desired properties, for example increased flameproofing or low shrinkage, additives are added. Thus, for example magnesium oxide increases the plasticity and kaolin increases the acid resistance.
Of course, the highest temperatures occur in the lighting unit. To date, the reflectors have therefore been produced either from sheet metal, from thermosetting plastics, such as BMC, or from metallized, injection-moulded amorphous high-Tg thermoplastics (PEI, PSU, PES). The high tolerance requirements coupled with the surface quality of the injection moulded parts which is required for metallization have been met to date mainly by filler-free amorphous high-Tg thermoplastics or coated thermosetting plastics.
An example of one of said high-Tg thermoplastics is the polyether sulphone ULTRASON E® from BASF Ludwigshafen, Germany (having an iridescence temperature of 212° C.), as described in the journal cited below. In the course of the progressive reduction of complexity, increasing integration of headlamp components to give highly developed lighting systems which are expected to permit higher material requirements is taking place at present (J. Queisser, M. Geprägs, R. Blum and G. Ickes, Trends bei Automobilscheinwerfern [Trends in automobile headlamps], Kunststoffe [Plastics] March 2002, Hanser Verlag, Munich).
The prior art moreover discloses compositions which comprise a fibrillar, inorganic filler (cf. EP 0 863 180) and an additional particulate inorganic filler (EP 1 312 647 or EP 0 585 056) or which comprise only a particulate, inorganic filler (EP 0 913 421). A material for the production of street light reflectors is known by the name MINLON® (E.I. du Pont de Nemours & Co., Wilmington, USA). Said product is the semicrystalline Nylon 66 (PA 66), which also comprises 36-40% of classical minerals in addition to a heat stabilizer. However, from the point of view of the surface quality, this material appears unsuitable, at least for vehicle lights. Here too, the considerable lengthening of the cycle times during injection moulding with such compositions compared with amorphous polymers is regarded as a further disadvantage.
A further requirement relates to the surface quality of the (generally curved) plastic surface to be coated. Especially in the case of reflectors, in which luminous efficiency is essential, a very smooth, highly glossy surface which is as homogeneous as possible must be provided for coating. Plastics which have poor flow or solidify too early or an addition of fillers often lead in the injection mould to a rough, matt or irregular impression, measured by the extremely high requirements of a mirror-smooth surface, even if the corresponding surface of the shaping mould is polished to a high gloss.
The prior art discloses further compositions without fillers. However, these likewise achieve only inadequate Tg values of less than 175° C. (cf. for example EP 0 313 436, EP 0 553 581 and U.S. Pat. No. 4,898,896). This category of polymers which are inadequate for the planned use also includes polyarylamides such as IXEF® 2057 (Solvay Advanced Polymers), polyarylates, polybutylene terephthalate (PBT, for example, ARNITE® TV4 220 from DSM).
The transparent, colourless and amorphous homopolyamides disclosed in the European patent EP 0 725 101 B2 have a glass transition temperature of about 157° C. and are at any rate suitable for the production of subreflectors, but unsuitable for the production of light-reflecting components which are exposed to operating temperatures of at least 200° C.
U.S. Pat. No. 6,355,723 B1 discloses injection moulded reflectors comprising amorphous thermoplastics, such as polyetherimides, polyaryl ethers, polyether sulphones, polysulphones, polycarbonates, polyestercarbonates, polyarylates, polyamides, polyesters and single-phase mixtures of said thermoplastics. These reflectors can be provided directly with a metal layer and have a glass transition temperature (Tg) of at least 170° C. to 200° C. In order to be able easily to detect any surface defects before the metallization of the reflector surface by means of visual inspection and to suppress undesired light effects due to unmetallized parts of the reflectors, all these reflectors are coloured black by admixing dyes.
However, owing to their excessively high coefficients of expansion, the polycarbonates or copolycarbonates previously described in the prior art have the disadvantage that they may have only limited suitability or even be unsuitable for use as a metallized component in, for example, high temperature applications as a reflector.
Polymer materials having very good optical and thermal properties are required for the use of flexible substrates for display applications, for example LCD or OLED displays. Thus, a sufficiently high glass transition temperature of the material is required for the production of the thin film transistor (TFT) elements on the substrates, for example by the a-Si:H method. Polyethylene naphthalate (PEN) has a low coefficient of thermal expansion but has only a low glass transition temperature of 120° C. and appears birefringent. Polyarylates (PAR) have a high glass transition temperature (215° C.) and an optically isotropic appearance but are very expensive materials. This also applies to polyether sulphone (PES, Tg=220° C.). Polyimides have the highest glass transition temperature (360° C.) in addition to very low coefficients of thermal expansion but seem orange to brown in optical appearance. The costs for these materials are likewise very high. For applications in display technology, very good optical properties, such as high transparency without birefringence, are additionally required. A polymer material which has very good transparency in combination with isotropic optical appearance at acceptable costs and at simultaneously high glass transition temperature and low thermal expansion would be desirable for display technology.
It was therefore the object to develop aromatic (co)polycarbonates which have reduced coefficients of thermal expansion and are simultaneously distinguished by outstanding adhesion to metal and good thermal properties (in particular high Vicat or glass transition temperature) in combination with a good surface (which is suitable for direct coating with a metal layer without additional pretreatment step) and can be prepared with good heat distortion resistance comparable to that of materials disclosed in the prior art.
The novel compositions should moreover show improved flowability.